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/*

   Copyright 2009-2013 Attila Szegedi

   Licensed under both the Apache License, Version 2.0 (the "Apache License")

   and the BSD License (the "BSD License"), with licensee being free to

   choose either of the two at their discretion.

   You may not use this file except in compliance with either the Apache

   License or the BSD License.

   If you choose to use this file in compliance with the Apache License, the

   following notice applies to you:

       You may obtain a copy of the Apache License at

           http://www.apache.org/licenses/LICENSE-2.0

       Unless required by applicable law or agreed to in writing, software

       distributed under the License is distributed on an "AS IS" BASIS,

       WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or

       implied. See the License for the specific language governing

       permissions and limitations under the License.

   If you choose to use this file in compliance with the BSD License, the

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       * Redistributions of source code must retain the above copyright

         notice, this list of conditions and the following disclaimer.

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*/

/**

 * <p>

 * Dynalink is a library for dynamic linking of high-level operations on objects.

 * These operations include "read a property",

 * "write a property", "invoke a function" and so on. Dynalink is primarily

 * useful for implementing programming languages where at least some expressions

 * have dynamic types (that is, types that can not be decided statically), and

 * the operations on dynamic types are expressed as

 * {@link java.lang.invoke.CallSite call sites}. These call sites will be

 * linked to appropriate target {@link java.lang.invoke.MethodHandle method handles}

 * at run time based on actual types of the values the expressions evaluated to.

 * These can change between invocations, necessitating relinking the call site

 * multiple times to accommodate new types; Dynalink handles all that and more.

 * <p>

 * Dynalink supports implementation of programming languages with object models

 * that differ (even radically) from the JVM's class-based model and have their

 * custom type conversions.

 * <p>

 * Dynalink is closely related to, and relies on, the {@link java.lang.invoke}

 * package.

 * <p>

 *

 * While {@link java.lang.invoke} provides a low level API for dynamic linking

 * of {@code invokedynamic} call sites, it does not provide a way to express

 * higher level operations on objects, nor methods that implement them. These

 * operations are the usual ones in object-oriented environments: property

 * access, access of elements of collections, invocation of methods and

 * constructors (potentially with multiple dispatch, e.g. link- and run-time

 * equivalents of Java overloaded method resolution). These are all functions

 * that are normally desired in a language on the JVM. If a language is

 * statically typed and its type system matches that of the JVM, it can

 * accomplish this with use of the usual invocation, field access, etc.

 * instructions (e.g. {@code invokevirtual}, {@code getfield}). However, if the

 * language is dynamic (hence, types of some expressions are not known until

 * evaluated at run time), or its object model or type system don't match

 * closely that of the JVM, then it should use {@code invokedynamic} call sites

 * instead and let Dynalink manage them.

 * <h2>Example</h2>

 * Dynalink is probably best explained by an example showing its use. Let's

 * suppose you have a program in a language where you don't have to declare the

 * type of an object and you want to access a property on it:

 * <pre>

 * var color = obj.color;

 * </pre>

 * If you generated a Java class to represent the above one-line program, its

 * bytecode would look something like this:

 * <pre>

 * aload 2 // load "obj" on stack

 * invokedynamic "GET_PROPERTY:color"(Object)Object // invoke property getter on object of unknown type

 * astore 3 // store the return value into local variable "color"

 * </pre>

 * In order to link the {@code invokedynamic} instruction, we need a bootstrap

 * method. A minimalist bootstrap method with Dynalink could look like this:

 * <pre>

 * import java.lang.invoke.*;

 * import jdk.dynalink.*;

 * import jdk.dynalink.support.*;

 *

 * class MyLanguageRuntime {

 *     private static final DynamicLinker dynamicLinker = new DynamicLinkerFactory().createLinker();

 *

 *     public static CallSite bootstrap(MethodHandles.Lookup lookup, String name, MethodType type) {

 *         return dynamicLinker.link(

 *             new SimpleRelinkableCallSite(

 *                 new CallSiteDescriptor(lookup, parseOperation(name), type)));

 *     }

 *

 *     private static Operation parseOperation(String name) {

 *         ...

 *     }

 * }

 * </pre>

 * There are several objects of significance in the above code snippet:

 * <ul>

 * <li>{@link jdk.dynalink.DynamicLinker} is the main object in Dynalink, it

 * coordinates the linking of call sites to method handles that implement the

 * operations named in them. It is configured and created using a

 * {@link jdk.dynalink.DynamicLinkerFactory}.</li>

 * <li>When the bootstrap method is invoked, it needs to create a

 * {@link java.lang.invoke.CallSite} object. In Dynalink, these call sites need

 * to additionally implement the {@link jdk.dynalink.RelinkableCallSite}

 * interface. "Relinkable" here alludes to the fact that if the call site

 * encounters objects of different types at run time, its target will be changed

 * to a method handle that can perform the operation on the newly encountered

 * type. {@link jdk.dynalink.support.SimpleRelinkableCallSite} and

 * {@link jdk.dynalink.support.ChainedCallSite} (not used in the above example)

 * are two implementations already provided by the library.</li>

 * <li>Dynalink uses {@link jdk.dynalink.CallSiteDescriptor} objects to

 * preserve the parameters to the bootstrap method: the lookup and the method type,

 * as it will need them whenever it needs to relink a call site.</li>

 * <li>Dynalink uses {@link jdk.dynalink.Operation} objects to express

 * dynamic operations. It does not prescribe how would you encode the operations

 * in your call site, though. That is why in the above example the

 * {@code parseOperation} function is left empty, and you would be expected to

 * provide the code to parse the string {@code "GET_PROPERTY:color"}

 * in the call site's name into a named property getter operation object as

 * {@code new NamedOperation(StandardOperation.GET_PROPERTY), "color")}.

 * </ul>

 * <p>What can you already do with the above setup? {@code DynamicLinkerFactory}

 * by default creates a {@code DynamicLinker} that can link Java objects with the

 * usual Java semantics. If you have these three simple classes:

 * <pre>

 * public class A {

 *     public String color;

 *     public A(String color) { this.color = color; }

 * }

 *

 * public class B {

 *     private String color;

 *     public B(String color) { this.color = color; }

 *     public String getColor() { return color; }

 * }

 *

 * public class C {

 *     private int color;

 *     public C(int color) { this.color = color; }

 *     public int getColor() { return color; }

 * }

 * </pre>

 * and you somehow create their instances and pass them to your call site in your

 * programming language:

 * <pre>

 * for each(var obj in [new A("red"), new B("green"), new C(0x0000ff)]) {

 *     print(obj.color);

 * }

 * </pre>

 * then on first invocation, Dynalink will link the {@code .color} getter

 * operation to a field getter for {@code A.color}, on second invocation it will

 * relink it to {@code B.getColor()} returning a {@code String}, and finally on

 * third invocation it will relink it to {@code C.getColor()} returning an {@code int}.

 * The {@code SimpleRelinkableCallSite} we used above only remembers the linkage

 * for the last encountered type (it implements what is known as a <i>monomorphic

 * inline cache</i>). Another already provided implementation,

 * {@link jdk.dynalink.support.ChainedCallSite} will remember linkages for

 * several different types (it is a <i>polymorphic inline cache</i>) and is

 * probably a better choice in serious applications.

 * <h2>Dynalink and bytecode creation</h2>

 * {@code CallSite} objects are usually created as part of bootstrapping

 * {@code invokedynamic} instructions in bytecode. Hence, Dynalink is typically

 * used as part of language runtimes that compile programs into Java

 * {@code .class} bytecode format. Dynalink does not address the aspects of

 * either creating bytecode classes or loading them into the JVM. That said,

 * Dynalink can also be used without bytecode compilation (e.g. in language

 * interpreters) by creating {@code CallSite} objects explicitly and associating

 * them with representations of dynamic operations in the interpreted program

 * (e.g. a typical representation would be some node objects in a syntax tree).

 * <h2>Available operations</h2>

 * Dynalink defines several standard operations in its

 * {@link jdk.dynalink.StandardOperation} class. The linker for Java

 * objects can link all of these operations, and you are encouraged to at

 * minimum support and use these operations in your language too. To associate

 * a fixed name with an operation, you can use

 * {@link jdk.dynalink.NamedOperation} as in the above example where

 * {@code StandardOperation.GET_PROPERTY} was combined with the name

 * {@code "color"} in a {@code NamedOperation} to form a property getter for the

 * property named "color".

 * <h2>Composite operations</h2>

 * Some languages might not have separate namespaces on objects for

 * properties, elements, and methods, and a source language construct might

 * address two or three of them. Dynalink supports specifying composite

 * operations for this purpose using the

 * {@link jdk.dynalink.CompositeOperation} class.

 * <h2>Language-specific linkers</h2>

 * Languages that define their own object model different than the JVM

 * class-based model and/or use their own type conversions will need to create

 * their own language-specific linkers. See the {@link jdk.dynalink.linker}

 * package and specifically the {@link jdk.dynalink.linker.GuardingDynamicLinker}

 * interface to get started.

 * <h2>Dynalink and Java objects</h2>

 * The {@code DynamicLinker} objects created by {@code DynamicLinkerFactory} by

 * default contain an internal instance of

 * {@code BeansLinker}, which is a language-specific linker

 * that implements the usual Java semantics for all of the above operations and

 * can link any Java object that no other language-specific linker has managed

 * to link. This way, all language runtimes have built-in interoperability with

 * ordinary Java objects. See {@link jdk.dynalink.beans.BeansLinker} for details

 * on how it links the various operations.

 * <h2>Cross-language interoperability</h2>

 * A {@code DynamicLinkerFactory} can be configured with a

 * {@link jdk.dynalink.DynamicLinkerFactory#setClassLoader(ClassLoader) class

 * loader}. It will try to instantiate all

 * {@link jdk.dynalink.linker.GuardingDynamicLinkerExporter} classes visible to

 * that class loader and compose the linkers they provide into the

 * {@code DynamicLinker} it creates. This allows for interoperability between

 * languages: if you have two language runtimes A and B deployed in your JVM and

 * they export their linkers through the above mechanism, language runtime A

 * will have a language-specific linker instance from B and vice versa inside

 * their {@code DynamicLinker} objects. This means that if an object from

 * language runtime B gets passed to code from language runtime A, the linker

 * from B will get a chance to link the call site in A when it encounters the

 * object from B.

 */

package jdk.dynalink;